Let me just say that my assumptions, irrespective of the data, are that the Nebulas are a nice way for SFWA members to log-roll each other, that the Hugo voters have long held noticeable collective biases that may imply cliquishness (the many nominations for Silverberg and Sawyer leap to mind), and that the SFSite poll seems to suggest more books I actually like, if only because it offers more options.

Anyway, here are the average Goodreads scores for novels that were Nebula nominees, Hugo nominees, and SFSite Readers’ Choice selections over the past ten years.

IMDb provides text file dumps of most of their public data. Below are this year’s films to date, ranked by the weighted IMDb score. I’ve included the current Rotten Tomatoes score for each.

Computing the correlation coefficient (r) for the raw, unweighted IMDb score vs. the Rotten Tomatoes score, I find it’s a .7. It goes down to .67 for weighted IMDb scores vs. Rotten Tomatoes, and that’s a weak correlation (r^2 = 0.45), indicating Rotten Tomatoes scores are not generally great predictors of what IMDb users will think and vice versa.

But if we consider only IMDb’s top 20 films for this year, the correlation improves greatly: r = 0.83; r^2 = 0.69. That’s a fairly strong correlation. Apparently, when IMDb users really like a film, Rotten Tomatoes critics do too. It’s the unpopular films that are a bit murkier.

Eyeballing the discrepancies at the lower end, it looks like Rotten Tomatoes may be a slightly better source for a neutral viewer trying to prioritize their movie choices. Both sources promote major franchises (Harry Potter 7.5, Fast 5, X-Men, etc.) as well as smaller, quieter films, but Rotten Tomatoes is usually more discriminating. IMDb users often give average ratings to films hated on Rotten Tomatoes (Transformers 3, Cars 2, etc.), but they never really love a film not also loved on Rotten Tomatoes. And the only film unpopular on IMDb that Rotten Tomatoes rates above 70% is Cedar Rapids. Where Rotten Tomatoes really goes wrong, though, is giving a “fresh” rating to films in the 60-69% positive range–those wind up all over the map in the IMDb ratings.

Verdict: to pick a current movie, just use Rotten Tomatoes and common sense. Focus on movies with lots of reviews that score 80%+ overall, and try not to waste time on anything that scores below 70% unless you feel pretty sure going in that it’s the perfect film for the occasion, in which case you may as well not consider any ratings at all.